This invention relates to a color Xerographic printing system having a multicolor printbar rather than a traditional polygon ROS (raster output scanning) system. More particularly, the present invention relates to a color Xerographic printing system having a polychromatic photoreceptor and a multicolor printbar for developing a full color image on the photoreceptor.
Generally, the process of Xerographic (electrostatographic) printing includes the step of charging an imaging member to a substantially uniform potential to sensitize the surface thereof. The charged portion of the surface is exposed to an image, such as an image of an original document being reproduced, or to a computer-generated image written by a raster output scanner. This records an electrostatic latent image on the imaging member corresponding to the original document or computer-generated image. The recorded latent image is then developed by bringing a developer material into contact therewith. In a tri-level system, three separate potential levels are used. The unexposed areas of the latent image are developed in one color, and the fully discharged areas are developed in another color; the partially exposed areas remain undeveloped. This forms a toner powder image on the imaging member that is subsequently transferred to a substrate, such as paper. Finally, the toner powder image is permanently affixed to the substrate in image configuration, for example by heating and/or pressing the toner powder image.
A suitable developer material may be a two-component mixture of carrier particles having toner particles triboelectrically adhered thereto. The toner particles are attracted to and adhere to the electrostatic latent image to form a toner powder image on the imaging member surface. Suitable single component developers are also known. Single component developers comprise only toner particles; the particles have an electrostatic charge (for example, a triboelectric charge) so that they will be attracted to, and adhere to, the latent image on the imaging member surface.
Various forms of systems for producing two-color developed images are also known. For example, U.S. Pat. No. 4,078,929 to Gundlach teaches the use of a tri-level electrostatographic system as a means to achieve singlepass highlight color images. Gundlach teaches a method for two-color development of an electrostatic charge pattern of a single polarity and having three different levels of potential by utilizing relatively negatively charged toner particles of one color and relatively positively charged toner particles of a second color. In this method, the photoreceptor is initially charged to a voltage V.sub.0. It is then selectively discharged with a single raster output scanner to approximately V.sub.0 /2 in the background areas and to near 0 or residual potential in the color areas. The fully discharged areas are printed in color, and the unexposed areas, which undergo dark discharge, are printed in black (or a second color). Alternatively, the colors may be reversed, i.e., the unexposed areas may be developed in color, and the areas of near 0 or residual potential may be developed in black (or a different color).
Another method of two-color reproduction is disclosed in U.S. Pat. No. 3,013,890 to Bixby. Bixby teaches a method in which a charge pattern of either a positive or negative polarity is developed by a single, two-color developer. The developer of Bixby comprises a single carrier that supports both triboelectrically relatively positive and relatively negative toner. The positive toner is a first color and the negative toner is a second color. The method of Bixby develops positively charged image areas with the negative toner and develops negatively charged image areas with the positive toner. A two-color image occurs only when the charge pattern includes both positive and negative polarities.
However, these development systems do not provide multicolor or full spectrum print results, desirable in many applications. These development systems rely upon a photoreceptor, charged to different charge amounts by a single charging means.
Various forms of development systems for producing multicolor developed images are also known. For example, U.S. Pat. No. 5,347,303 to Kovacs et al. discloses a full color xerographic printing system, in either single or double pass operation, using a single polygon ROS containing a dual wavelength diode laser. The system further includes a dual layer photoreceptor wherein each photoreceptor layer is sensitive to or accessible by only one of the two wavelengths of the diode laser.
U.S. Pat. No. 5,373,313 to Kovacs discloses a full color xerographic printing system, in single pass operation, using a single polygon ROS containing a multiple wavelength diode laser. The system further includes a multiple layer photoreceptor wherein each photoreceptor layer is sensitive to or accessible by only one of the multiple wavelengths of the diode laser.
U.S. Pat. No. 5,444,463 to Kovacs et al. discloses a single pass three-toner color xerographic development system. The development system uses a two-layer photoreceptor and a single polygon ROS system having a dual wavelength diode laser array. Each of the two photoreceptor layers is sensitive to or accessible by only one of the dual wavelengths of the diode laser.
There remains a need in the art for multicolor printing systems that can operate at higher speeds, but also at a lower cost. However, production of such systems has heretofore been hindered by the difficulty in manufacturing the necessary multicolor laser diode array. For example, due to absorption spectra of polychromatic photoreceptors for the development of four colors, the multiple wavelengths need to be separated by a large amount, typically 100 nm or more. Thus a multicolor system would be expected to require wavelengths of in the blue (400 to 500 nm), green (500 to 600 nm), red (600 to 700 nm) and infrared (700 to 800 nm) regions. However, the attainment of four wavelengths over such a wide range is very difficult in a monolithic diode laser structure, because it requires the integration of at least two different material systems. In addition, printing at high speed requires an addressable array at each color, making the array even more complex and expensive to fabricate. Nonmonolithic assemblies of addressable arrays are another possible approach to fabricating such a monolithic laser array source, but such assemblies are not straightforward and have not yet been demonstrated.
Therefore, there remains a need in the electrostatographic printing art for a full color printing system that does not require the complex ROS and multiple wavelength laser systems. In particular, there is a need for a less complex and less costly system for high speed multicolor printing.